A displacement measuring device described Japanese Laid-open Patent Publication No. H09-267418 scans a light beam irradiated to an object to be measured and then measures a distance up to a displacement surface of the object with respect to a reference surface of the object based on triangulation.
However, the device is not suitable for such purposes as, for example, tracing the locus of motion of a physical object in an object space. Because it is necessary to scan the whole object space with the beam in order to trace the locus and comparative long time is taken until the device finishes scanning the whole object space with the beam.
This problem can be solved by a range image sensor according to our another invention (cf. Japanese Patent Publication No. 2004-272001). The sensor comprises a light source which emits intensity modulated light toward an object space and a light detecting element with a photosensitive array disposed to face the object space. While the intensity modulated light is emitted toward the object space, a range image is constructed based on the output of the light detecting element. When at least a physical object is in the object space, each image element in the range image has a corresponding distance value up to the physical object. Each distance value is obtained from a difference between the phase of the light emitted from the light source and the phase of the light received by a corresponding pixel of the light detecting element. A period of time (hereinafter referred to as an “integration period”) during which each pixel mainly integrates an electric charge in response to the light is set to a shorter time than one period of the intensity modulated light.
If the integration period is comparative long time, it is possible to reduce shot noise occurred at the photosensitive array to raise the S/N ratio. But, the light detecting element becomes easy to be saturated under circumstances which includes a lot of ambient light noise (e.g., solar radiation). On the other hand, if the integration period is comparative short time, the light detecting element becomes hard to be saturated. However, the S/N ratio falls and it is difficult to obtain correct distance.